The invention relates to high strength, low weight laminar structures, and more particularly to corrugated, undulant or otherwise three-dimensional cores for such structures. Heretofore it has been known to take rigid corrugated substances and to form laminates therefrom by fixing sheets to the faces of the corrugated substance. Such structures have been limited in the configurations available because the cores of corrugated material of requisite strength have been fixed in their attitude and shape at the time the outer sheets are applied. I have invented cores and process and apparatus for making cores which afford a configurable core with great flexibility in one direction and limited flexibility in the other direction which is a stable three dimensional element after curing.